net: mvpp2: add RXQ flow control configurations

This patch adds RXQ flow control configurations.
Flow control disabled by default.
Minimum ring size limited to 1024 descriptors.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Acked-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>